1. Field of the Invention
The present invention relates to a telephone apparatus, and more particularly relates to a telephone apparatus to which a plurality of telephone lines are connected and which has a transfer function at the time of a call between telephone sets.
2. Description of the Related Art
In general, a telephone apparatus with a transfer function is connected to a plurality of telephone sets as shown in FIG. 3 in the case where it is used in a tandem manner in an office. The operation of a telephone apparatus S when a telephone set P calls to the telephone apparatus S will be explained. When a dial signal based on a phone number which designates the telephone apparatus S is output from the telephone set P, a switching system Ex1 sends a ring signal to the telephone apparatus S, so that a speech path is formed between the telephone set P and the telephone apparatus S.
For the transfer, the telephone apparatus S sends a dial signal which designates a telephone set Q, so that a speech path is formed between the telephone apparatus S and the telephone set Q. Moreover, a speech path is formed between the telephone set on the side of a calling person (i.e. the telephone set P) and the telephone set on side of a called person (i.e. the telephone set Q). In other words, the telephone set P on the side of the calling person is connected to the telephone apparatus S with a transfer function through a switching system Ex2 and a switching system Ex1 by a public telephone line, and the telephone set Q on the side of the called person is connected to the telephone apparatus S with a transfer function through a switching system Ex3 and the switching system Ex1 by a public telephone line. A call between the telephone set P and the telephone set Q is produced by connecting the telephone set P and the telephone set Q with the telephone apparatus S.
By the way, the level of the call between such line wires becomes smaller than that of a call between two persons due to the circuit loss between the switching system Ex1 and the telephone apparatus S. If the damped signal is amplified by an amplifier, a loop is formed. Therefore, the signal return to the amplifier occurs, causing oscillation.